Growing physically and aging are two different things. Cadance was the youngest Alicorn, probably born/created near the same time as Shining armor, but you'll notice how big Celestia is compared to everyone else. this means that while Twilight, cadence, and even Luna still have a was to grow, they will not physically appear to age past their prime years.

Agree a reality that is switched around by a little Discord. Where all old are you "Good grief Granny Apple" and young are old "Hmmm would the crusader's have there marks?" Lol that would be an interesting show if not a bit slow. Lol

Correction. Celestia and Luna have lived for thousand of years, and they are pretty much Goddesses. Currently, there are only 4 canon alicorns on the show (one half also appears radically different than the other), so there is little evidence to suggest that "normal" alicorns like Twilight and Cadance have immortality or even an extended lifespan like Celestia and Luna. For all we know, the extent of extra abilities bestowed on Twilight by her newfound alicorn status may simply be the ability to fly.

Nah, I have theory about what Alicorn is base on this review: [link]I made a theory, in my headcanon, Alicorns are not born but they are ponies turned into Alicorns once they create a new magic or gain ultimate knowledge.

Twilight turned into Alicorn once she created magic of friendship, and it make sense since in Equestria emotions are powerful force, for example, elements of harmony are powerful, changelings feed on love, Sombra used dark magic with hate and anger, so when Starswir tried to create spell to become Alicorn and failed, Celestia trained and tested Twilight so she can finish shell ( Twilight had friends and elements of harmony and this helped her to create new magic and finish spell ).

And here is my theory about Cadance, she was Crystal Pony, Crystal ponies have no wings and no horn, so they can not do magic, but Cadance created Crystal Heart, an artifact what use love magic from Crystal Ponies, and this turned her into Alicorn, and Crystal Ponies choised her to be they princess ( Cadance have Crystal Heart as cutie mark ), after Sombra took over, Cadance run to Equestria to get help, but Sombra vanished with empire, and Cadance was stuck in Canterlot, she was princess but had no power since all her subjects were gone.Celestia and Luna took care of her ( just Celestia after Nightmare Moon incident ), but Cadance not wanted to be a burden so she took small jobs to gain bits and pass time, that is why she was babysitting Twilight as she waited for her Empire to return.

Luna probably turned into Alicorn when she created a dream magic what let her enter ponies dream, but I do not have enough info to make theory on Celestia and Luna, at last not until season 4.

The only think I can not explain is why Cadance was so small when she babysit Twilight, but I think it was just animator mistake.